Wafers are typically diced on adhesive tape or plastic foil so the dice are contained, but easily removable, e.g., for storage in waffle packs, gel pack, tape and reel, or bonding into a package. Individual dice may be removed in various manners, e.g., gripping the die and pulling it from the tape, pushing the die from the tape with a pin, or drawing the tape away from the die by vacuum. The released die is generally picked by a vacuum tool and placed as desired. Some systems include a pick and place machine and die bonder equipped with an ejector needle system that pushes against the die back side to help break the adhesion force of the adhesive foil, and a vacuum pick-up tool with coordinated z-axis movement that holds the die before transfer to bond position.
The picking of dice from the wafer is typically facilitated using a wafer map indicating the positions of the various dice on the wafer. In a conventional process, one or more reference dice are used to align the pick position, i.e., the position of the picking tool, e.g., die attach (D/A) machine, relative to the wafer or particular die.
FIG. 1 shows a schematic plan view diagram of a semiconductor integrated circuit wafer comprising a plurality of semiconductor dice. A silicon wafer 102 may be scribed into a plurality of semiconductor dice 104 for further processing to create planar transistors, diodes and conductors on each of the plurality of semiconductor dice 104. After all circuits have been fabricated on the plurality of semiconductor dice 104, the dice 104 are singulated (separated) and packaged into integrated circuits (not shown).
FIGS. 2A1-2C2 illustrate a conventional picking process using a reference die for alignment. In particular, FIGS. 2A1-2C2 illustrate a physical wafer 102 and a corresponding wafer map 110 for three steps of the process. The wafer 102 may be pre-fabricated with one or more reference dice, which may be arranged in groups. In this example, wafer 102 is pre-fabricated with two groups of reference dice: an upper reference die group 105A and a lower reference die group 105B, each including the same number of dice (in this example, five dice) aligned in a row. In a first step, with reference to FIGS. 2A1 and 2A2, user may select one of the reference die groups (reference die group 105A or reference die group 105B) to act as a spatial reference for subsequent die picking. The individual die 104 on each end of the selected reference die group 105A or 105B (in the illustrated example, the left-most die 104 and right-most die 104 of the selected reference die group 105A or 105B) may be visually marked by the software, e.g., by adding a “+” or other symbol over such dice 104 on the wafer map 110. As used herein, the selected reference die group (here, either group 105A or 105B) is also simply referred to as the reference die 105.
In a second step, illustrated by FIGS. 2B1 and 2B2, a picking/die attach machine begins picking up dice 104 from any position on the wafer 102, while using the selected reference die 105 to check the relative positioning of the picking machine and wafer 102. The picking process may continue until all or substantially all dice 104 on the wafer 102 are consumed, as shown in FIGS. 2C1 and 2C2.
However, the selected reference die 105 typically provides only two points (the individual dice 104 at each end of the reference die 105) in the wafer 102 such that the picking tool/die attach machine often cannot detect incorrect alignment relative to the wafer 102. Thus, the picking/die attach machine may slip one row and/or column, which may lead to low yield at a final test due to failed test dice being assembled at the die attach.